Sex might be fun but it's not without risks. As your partner exposes themselves to you, they also expose you to whatever bacteria, viruses or parasites they might be carrying. But some animals have a way around that. Ekaterina Litvinova has found that when male mice get a whiff of female odours, their immune systems prepare their airways for attack, increasing their resistance to flu viruses.
Litvinova worked with a group of mice that were exposed to bedding that had previously been soiled by females in the sexually receptive parts of their cycle. She compared them to a second more monastic group that were isolated from female contact.
Male mice use smells to track down females who are ready to mate. They'll follow markings of faeces and urine and when they actually find the female, they'll continue sniffing her nose and genitals. Each of these nasal encounters could be a source of infection. She then pitted both groups against a flu virus. Influenza doesn't affect wild populations of house mice, so the virus in this case is acting as more of an indicator of the animals' defences, rather than a representative of a real threat.
Both groups of mice lost a bit of weight, but at certain doses of virus, those that had been exposed to female aromas kept more of their grams on. They also fared better in the long run - just 20% of them died, compared to 46% of those that had only smelled male odours.
Their lungs revealed the secret behind their resistance. After sniffing the female bedding, the males had conscripted white blood cells to their airways - some of the most potent defences against respiratory infections. This flood of new recruits led to a defensive force almost three times as strong as those marshalled by isolated males. The arrival of white blood cells into the lungs can sometimes cause inflammation, which hampers a mouse's lung capacity. But Litvinova found that the bedding sniffers weren't affected in this way. Their aerobic abilities were normal.
Of course, mouse bedding is a smorgasbord of different chemicals and it isn't exactly free of infectious viruses and bacteria itself. Perhaps when males sniffed the bedding, these resident microbes triggered an immune response that allowed them to resist the later flu attack? Litvinova doesn't rule out this possibility, but she thinks that it's an unlikely explanation. After all, the male mice were all housed in groups. Even those that were isolated from females still got to sniff the bedding of other males, and they weren't significantly protected against the flu.
Instead, she thinks that it's a sensible strategy for mice to get an immune boost if they smell the scent of a sexually receptive female. With mating and close-quarters contact on the cards, it's worth giving yourself better odds of warding away any pesky infections that might transfer alongside those bodily fluids. However, the best test of this hypothesis is one that hasn't been carried out yet - to see if males get different degrees of protection if they smell females at different parts of their sexual cycle.
Reference: Litvinova, E., Goncharova, E., Zaydman, A., Zenkova, M., & Moshkin, M. (2010). Female Scent Signals Enhance the Resistance of Male Mice to Influenza PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009473
Photo by Rama
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What happens when mice are housed communally? According to this hypothesis the males would exhibit chronic hyper immune activity. That should be easy to determine. Even with divided housing the bedding would be same, and if it is a chemical, and not microbial, response you should see the same result. If the immune response cycles with the female's receptivity, you have your answer.
A better test would be to isolate the causative agent "mouse bedding is a smorgasbord of different chemicals and it isn't exactly free of infectious viruses and bacteria itself"
The best part of further research would be to see if a similar thing happens in humans! Let's get some ethics approval and DO IT!
Remember, there are three Fs that stress readies us for. Not only fight and flight but also fornicate. Furthermore, the fight can occur at the microscopic level.
I would suggest that as the male mouse goes into "heat" an increase in dopamine levels would occur as they do in other anticipatory conditions. Devoino showed as early as 1984 that dopamine stimulates cytokines associated with the cellular immune system (Th1) while serotonin suppresses them. She could not use current terminology for it had not yet been described and her concepts of the thymus and adrenal may be more Selye than 2010.
The rat would be primed to combat viral and bacterial infections. Of course, if there is excessive allostatic load, the immune system falls apart. Best example, AIDS.
As Ader, Cohen and many others have shown, one can not separate emotions from the immune system. IMHO, every psychotropic is an immunomodulator.